1. Field of the Invention
This invention relates to a process of gasifying fine-grained solid fuels to produce a product gas which contains hydrogen, carbon oxides, and methane, in two interconnected gasifying stages by a treatment with at least one of the gasifying agents consisting of steam, oxygen and carbon dioxide under a pressure in the range from 2 to 150 bars and at temperatures of 500.degree. to 1500.degree. C. Usually the pressure in the gasifying stages is at least 5 bars and preferably in the range of 10 to 60 bars.
Solid fuels which may be used in the process include particularly coal and lignite as well as peat. The fuel subjected to the gasifying treatment has particle sizes below 3 mm. Oxygen may be used as a gasifying agent in known manner as commercially pure oxygen, air, or oxygen-enriched air. At the relatively high gasifying temperatures, carbon dioxide acts also as an oxidizing gasifying agent.
2. Discussion of Prior Art
From German (BRD) Offenlegungsschrift No. 2,511,191 it is known that carbonaceous material can be gasified in an arrangement in which two interconnected circulating fluidized beds are used. Solid and/or liquid fuel together with steam is charged to the first gasifying stage, which comprises a circulating fluidized bed. The solids left after the reaction in this stage enter in part the second circulating fluidized bed, in which a reaction in the presence of oxygen-containing gas and steam is effected at higher temperatures than in the first stage. The residual solids from the second gasifying stage are returned to the first stage and ensure sufficiently high temperatures therein. Having a high ash content and a low carbon content, these solids fed from the second stage to the first introduce considerable amounts of non-gasi-fiable material into the first stage. Additionally the solid fuel left after the treatment in the second gasifying stage reacts only so slowly that it can hardly be reacted in the first gasifying stage, where the temperatures are lower than anywhere else in the process. For this reason, the desired reaction between the fresh solid fuel and the gasifying agent in the first stage may cease or may be strongly adversely affected.